Regulations require that the fuel tanks of civil aircraft are rendered inert, that is to say that the flammability hazard posed by the fuel tank is reduced, by maintaining the oxygen concentration below a pre-set limit—typically 11.9% oxygen by volume at sea level but can vary from 9% to 12%. It will be appreciated that an inflow of gas into the aircraft fuel tank is required both to make up for the burn rate of the fuel exiting the tank, and also to maintain the pressure differential across the tank within the structural design limits during descent. The mass flow rate required during descent is therefore relatively high as a substantial mass is required to pressurize the tank.
Aircraft are fitted with systems to reduce the flammability of the mixture of gas and fuel vapour that exists in the (ullage) space above the liquid fuel within the tanks. A means of flammability reduction is to replace the air within the ullage space with Nitrogen Enriched Air (NEA), in which the concentration of nitrogen is greater than ambient air and the concentration of oxygen is lower.
A preferred type of system is the “continuous flow” as opposed to the store gas system. In the continuous flow system type, the NEA is generated on demand by an air separation device. In stored gas systems, nitrogen or NEA is held in storage vessels as compressed gas or by physical adsorption in a medium.
The continuous flow type of system tends to be lighter and less complex than the stored gas class of system The size of a continuous flow system may be minimised by filling the fuel tanks with NEA having a low concentration of oxygen prior to the period of descent or rapid descent and supplying sufficient NEA to the tanks to ensure that the bulk average of the oxygen concentration at the end of descent is less than the pre-set limit. A problem with this sizing condition is that in the case where the flow of NEA is insufficient to prevent the inflow of air, the air enters the tank at one or more vent valves (typically two per tank). Consequently, in the zones local to the vent port or valve, the oxygen concentration is greater than the pre-set limit, creating an ignition “pocket”. Such a pocket tends to be concentrated by the wing ribs or any internal structure or baffles.